Where a cross-linked material with hydrophilic properties is required, traditionally silica is provided, whether as a coating or as a material as such. Rather than using solid SiO2 crystals, a polymeric network of the formula
is formed in situ as a polymer of silicic acid. Furthermore, chemical modification of the (usually mesoporous) silica surface offers ample scope for development of materials with pre-assigned properties for various applications.
An example of a current application of silica in order to provide a hydrophilic surface is engineering the properties of nanoparticles. A respective surface pretreatment enables the nanoparticles to be added as filler particles to a polymer, thereby forming a polymer-based nanohybrid. The surface treatment improves in these applications the bonding between filler particles and matrix material. Such engineered nanohybrid materials have become crucial for a number of applications in order to replace expensive metals and ceramics.
Materials constructed from particles below a critical dimension in width display unique chemical, physical and structural properties that differ significantly from macroscale and even atomic scale materials. Particles and tubes of nanosize have therefore become an attractive tool in various applications, in particular where they can be engineered to have specific properties to serve desired functions, such as medical applications. Desired properties of particles of nanosize are usually at least one of increased modulus and strength, transparency, decreased gas permeability, increased scratch, abrasion, heat resistance and decreased flammability.
Furthermore a downside of particles of nanosize is their handling in solution, in particular the handling of metal or metalloid particles (e.g. quantum dots) in aqueous solution. Aggregation and coagulation, as well as “salting out” are a serious problem (see e.g. Mulvaney, P., et al., J. Mater. Chem. (2000) 10, 1259-1270). Accordingly, surface treatments, for example with capping agents are commonly used for such applications as well.
Research efforts are currently aiming at optimizing silica coatings for surfaces, in particular nanoparticles and stabilizing sols during the preparation of silica coatings. A synthesis of gold-silica core-shell nanoparticles with tailored surface properties has also been reported recently. The common method of silica-coating of metal (e.g. gold) particles consists of three steps: (1) modification of the nanoparticle surface to make it vitreophilic (“metal oxide loving”) by using silane coupling agents having an amino or thiol group such as 3-aminopropyl-trimethoxysilane and 3-mercaptopropyltrimethoxysilane (MPS), (2) slow silica eposition in water from a sodium silicate solution, and (3) extensive growth of the silica shells through sol-gel reaction of silicon alkoxide in ethanol/ammonia mixtures.
Most currently available processes thus require the particle materials to be made in situ by a catalyzed (typically base catalysed) condensation of a sol gel reaction or by a precipitation reaction before a subsequent coating step since this procedure is accompanied with the lowest particle aggregation in the art. However, this procedure greatly limits the application potential of the composite due to the complex nature of the synthesis process that results in either very low productivity or lack of consistence.
Furthermore currently most commercially available nanomaterials come in the form of dried powders or are re-dispersed from the dried state. This inevitably results in the formation of both relatively loose agglomerates—due to electrostatic or Van-der-Waals forces between individual particles—and tighter aggregates, due to tight bonding that is hard to break. Therefore mechanical processing is necessary to disrupt agglomerates and aggregates into smaller units or individual particles. Dispersing respective nano-scale particles accordingly has to include aggregate/agglomerate disruption and stabilization of the dispersion.
Particles treated through regular wet chemical process are currently dispersed by stirring in water or organic solution. This simple process is however insufficient in overcoming agglomeration since the intensity of the force applied is too weak. Furthermore a common reaction process using a silane takes hours in dispersion before it has completely reacted with small particles. This difficulty is particularly significant where a commercially available dried powder is to be treated. Application of certain force is thus needed to keep particles from agglomerating throughout the surface treatment process.
Accordingly it is an object of the present invention to provide an improved coating method that can be used for surface coating of particles.